In recent years, a digital camera function has come to be incorporated in the mobile equipment such as a mobile phone. Furthermore, such sophisticated functions as auto-focusing function and handshake compensation function have also come to be mounted thereon. This trend has required the development of a microminiaturized actuator for moving a pickup lens and image pickup device. Thus, various forms of actuators to take the place of the conventional motor and gear train have been proposed. Of these, the actuator capturing the spotlight of the industry is the SMA actuator using a shape memory alloy (hereinafter referred to as “SMA”).
The following briefly describes the basic arrangement and operation of the SMA actuator with reference to FIGS. 4a, 4b and 4c, wherein an example is taken from the conventional SMA actuators. FIGS. 4a, 4b and 4c are schematic diagrams showing the basic arrangement and operation of the conventional SMA actuator.
In FIG. 4a, the SMA actuator 1 contains a drive member supporting member 11, fixing portions 13 and 15 as part of the supporting member 11, drive member 17, moving member 21, protrusion 23, bias member 31 and bias member fixing portion 33. The bias member 31 and bias member fixing portion 33 serve as a bias section of the present invention.
The moving member 21 that moves a driven member by mounting the driven member (not illustrated) or by engagement with the driven member is biased by a bias member 31 made up of a spring and others in the direction of being pulled by the bias member 31, namely, toward the right in the drawing, and is connected to the bias member fixing portion 33. In the meantime, the drive member 17 made up of SMA wire is extended between the protrusion 23 arranged on the moving member 21 and the drive member supporting member 11 arranged in the direction face to face with the bias member 31 with the moving member 21 kept sandwiched in-between, so that tension is applied to the moving member 21 in the direction against the biasing force of the bias member 31, namely, toward the left in the drawing. The drive member 17 is secured onto the drive member supporting member 11 at the fixing portions 13 and 15. The moving member 21 stops at the position where the biasing force of the bias member 31 is balanced with the tension of the drive member 17.
In FIG. 4b, when an electric current is applied to the drive member 17 in the direction marked by an arrow 41, the drive member 17 generates Joule heat by its own resistance, and is transformed in the state of higher elastic coefficient. In this case, the drive member 17 shrinks to get back to the length memorized at the time of high temperature due to the properties of SMA. Then tension is applied to the moving member 21 in the leftward direction through the protrusion 23. This tension causes the moving member 21 to move to the left against the biasing force of the bias member 31 by the distance indicated by arrow 43 of the drawing.
In FIG. 4c, when application of current to the drive member 17 has been suspended, the drive member 17 stops heat generation, and the drive member 17 gets back to the state shown in FIG. 4a by natural cooling through ambient air. The moving member 21 is shifted to the right by the biasing force of the bias member 31 by the distance indicated by the arrow 45 of the drawing, so that the moving member 21 gets back to the state of FIG. 4a. Repetition of the aforementioned procedure shifts the moving member 21 in the lateral direction.
In the SMA actuator having the aforementioned structure, the following technique has been proposed (e.g., Unexamined Japanese Patent Application Publication No. 2006-337533. The portion of the moving member in contact with the SMA is melted indirectly by heating the SMA at the portion in contact with the moving member, so that the SMA penetrates into that portion, whereby positions between the moving member of the SMA actuator and SMA are adjusted.
As schematically shown in FIG. 4c, the conventional SMA actuator has the following problem: A short circuit may be established between the protrusions 17a from the fixing portions 13 and 15, wherein these protrusions are the portions left behind after cutting the drive member 17 produced at the time of manufacturing the actuator. As a result, the drive current cannot be applied. Alternatively, a short circuit may be established with the circuit board such as the drive circuit of the SMA actuator (not illustrated). This may cause a trouble.